BUL 926 Understanding Factors Controlling Ammonia Volatilization from Fertilizer Nitrogen Applications Introduction NITROGEN (N) IS THE ESSENTIAL SOIL MINERAL Biswanath Dari Postdoctoral Fellow, Barley Agronomy NUTRIENT needed in the greatest quantity by plants and University of Idaho, Plant Sciences is a primary component of biological cycles. While N makes up 78% of the atmosphere, few plants (for instance, Christopher W. Rogers legumes) are adapted to convert or “fix” N directly from the Barley Agronomy atmosphere to satisfy their need for N. Thus, plants rely on University of Idaho, Plant Sciences available forms of N (ammonium; NH4 and nitrate; NO3) Olga S. Walsh from mineralization of organic soil N or the application Cropping Systems Agronomy of fertilizer N to optimize their growth and development. University of Idaho, Plant Sciences Crop production removes soil nutrients when crop outputs such as grain, straw, tubers, etc., are removed at harvest. Contents The primary forms of N found in N fertilizers are 1 Introduction ammonium (NH4), nitrate (NO3), and urea (CO(NH2)2) or combinations thereof. Plant availability and recovery 2 Sources of Ammonia Volatilization of N from NH4 or NH4-forming fertilizers are reduced 2 Factors Affecting Ammonia by N losses via leaching and runoff, denitrification, and Volatilization ammonia (NH3) volatilization. Gaseous N loss via NH3 4 Summary volatilization is a major potential pathway of loss (Fig. 1). Therefore, NH3 volatilization can potentially reduce a grower’s economic return and have negative impacts on the environment. Ammonium-forming fertilizers are susceptible to NH3 loss, especially when left on the soil surface. Under specific conditions, NH3 volatilization from urea and NH4-forming fertilizers has been reported to account for up to 50% of total N loss in agricultural systems (Lightner et al. 1990; Hamid and Mahler 1994; Eckard et al. 2003; IPNI 2011). The extent of N loss is dependent on a variety of factors including weather conditions, soil characteristics, and management. Thus, to maximize a cropping system’s sustainability, the mechanisms and factors that affect NH3 volatilization need to be understood and addressed through appropriate management practices. Figure 1. Schematic of nitrogen cycling including ammonia Figure 2. Change in ammonia (NH3) and ammonium ion (NH4+) volatilization in the soil–plant–atmosphere environment. concentration ratio as a function of pH. Sources of ammonia Factors affecting ammonia volatilization volatilization Ammonium or NH4-forming fertilizers such as urea Nitrogen losses via ammonia volatilization (46-0-0), ammonium sulfate (21-0-0-24), ammonium following surface application of NH4 or NH4- nitrate (34-0-0), and urea ammonium nitrate (UAN, forming N fertilizers are controlled by various soil, 28/32-0-0) are susceptible to NH3 volatilization losses. environmental, and fertilizer management factors as Agricultural systems are estimated to account for discussed in the following sections. approximately 89% of total NH3 volatilization in Soil factors North America (FAO 2014). Although animal produc- tion accounts for the major share of the total agricul- The extent of NH3 volatilization is influenced by soil ture-related NH3 volatilization, N loss from fertilized pH, type (or texture), cation exchange capacity (CEC), fields also plays a pivotal role in NH3 volatilization. soil organic matter (SOM), temperature, and moisture content of the soil. Ammonia volatilization occurs as NH4 is converted to NH3 gas at the soil surface and transported to the atmo- Soil pH sphere. Urea accounts for about 50% of the total world In general, alkaline soils (pH >7.0) are more suscepti- N fertilizer consumption and is the primary N fertilizer ble to NH3 volatilization, while neutral to acidic soils used in agriculture throughout the United States (FAO (pH 5.0–7.0) result in reduced NH3 volatilization loss- 2014). The pathway by which ammonia volatilization es. During urea hydrolysis, the initial soil pH increas- occurs from urea is shown in Equations 1–3. es surrounding the fertilizer. The ratio of NH4 to NH3 When either granular or liquid urea is applied to the is largely dependent on pH (Fig. 2), thus increased pH soil, it hydrolyzes and is converted to ammonium car- above 7.0 promotes NH3 volatilization. Fertilizer N bonate by the urease enzyme naturally occurring in sources such as urea can result in elevated pH condi- soils. Then, a portion of ammonium is converted into tions at the site of fertilizer contact with the soil even ammonia gas which can be lost to the atmosphere. in neutral to acidic soils, increasing NH3 volatilization risk. The ability of a soil to resist an increase in pH (that is, its buffering capacity) affects the amount of NH3 loss because of the buffering capacity’s effect on the ratio of NH3 to NH4 (Fig. 2). Soil physical and chemical properties Ammonia volatilization largely depends on the texture of a soil along with its buffering capacity. Generally, soil texture is an important indicator of 2 CEC since clay content is positively correlated with Research from Montana reported less than 10% CEC. A soil with a higher buffering capacity (because NH3 volatilization losses occurred within 2 weeks of of high CEC, high clay content, high SOM, or a surface application of urea to a dry soil. Additionally, combination of those) has a reduced rate of NH3 loss a wet soil surface can result in greater risk for NH3 because rapid pH change is restricted or because of loss even at a low soil temperature (i.e., 25 to 40°F). increased adsorption of NH4 to clay particles. For This particular situation may prevail especially instance, sandy soils with lower CEC and SOM, low during late fall, winter, and early spring fertilizer pH, low buffering capacity, and greater magnitude applications in Idaho. of pH changes can lead to greater NH3 volatilization Fertilizer management factors loss as compared to silty or clay soil with a higher CEC, SOM, and thus, buffering capacity. Ammonia volatilization is affected by fertilizer source, type, time, and method of application. Surface Soil temperature application of urea or other NH4-forming fertilizers Increased soil temperature increases the solubility is a predominant cause of NH3 volatilization of urea and thus, the rate of NH3 volatilization. from Idaho cropping systems. Broadcast urea Generally, the risk of NH3 volatilization is increased and other granular or liquid-N fertilizers without at soil temperatures over 70°F. However, the rate incorporation increases the susceptibility to NH3 loss. of NH3 volatilization can still be substantial at This results from greater exposure of the fertilizer near-freezing temperature. Specifically, a study to the combination of environmental conditions from Montana revealed that significant NH3 losses conducive to NH3 volatilization. Conversely, sub- (almost 40% of applied N) from surface-applied urea surface fertilizer banding helps to minimize NH3 can occur even when soil temperatures are cold volatilization risk on calcareous soils. (<5°F) (Jones et al. 2013). Another study by Engel et al. (2011) reported that approximately 12% of Environmental factors applied N was lost within 5 weeks of application Urea needs to be hydrolyzed before NH3 can be vol- when the soil temperature was 30°F even when the atilized and the rate of the hydrolysis process can be soil was covered with snow in December. Therefore, affected by certain environmental conditions such as soil surfaces can be subject to NH3 volatilization soil temperature and humidity. Weather conditions losses even if cold or frozen at the time of fertilizer such as rainfall, snowfall, or fog directly affect NH3 N application. volatilization loss from a soil. Fertilizer application should occur onto a dry soil surface where incor- Soil moisture poration into the soil can occur, and ½ inch of rain Soil moisture plays a critical role in determining should be sufficient to incorporate surface-applied the fate of applied fertilizer N. For example, urea fertilizer N (Jones et al. 2013). This will reduce NH3 applications to a wet soil surface result in increased volatilization substantially as compared to surface NH3 volatilization as the rate of hydrolysis is applications without incorporation. increased and movement into the soil is decreased The loss of NH3 from surface-applied N fertilizers due to the amount of water-filled pore spaces. increases in windy conditions, when NH3 is moved Similarly, prolonged contact of fertilizer granules into the atmosphere as the surface of the soil with a damp soil surface promote N loss. The dries (Cornell Univ. Coop. Ext. 2013). Generally, increased NH3 volatilization is related to urea being higher air temperature (>70°F) also increases highly hygroscopic (that is, it absorbs water from NH3 volatilization risk due to the increased speed both liquid water and water vapor in the air). As of hydrolysis of urea, and thus, greater NH3 a result, granular urea dissolves when in contact volatilization occurs under warm, windy conditions. with moisture and can be lost via volatilization Fluctuations in volatilization potential have also shortly after hydrolysis occurs following surface been noted throughout the day—the risk is generally applications (Wahl et al. 2006). On the other hand, greater during the warmer part of the day compared the application of fertilizer N to a dry soil with low to the night. relative humidity reduces NH3 volatilization losses. 3 Summary Engel, R., C. Jones, and R. Wallander. 2011. Ammonia Volatilization from Urea and Mitigation by NBPT Urea and other NH4-forming fertilizers are Following Surface Application to Cold Soils. Soil Sci. Soc. commonly used to optimize crop production, but are Am. J. 75:2348–2357. susceptible to losses as NH3 gas, particularly when International Plant Nutrition Institute (IPNI). 2016. Nitrogen left on the soil surface after application. Ammonia Notes: Ammonia Volatilization, Bulletin #6. Available volatilization results in loss of applied nutrients, from http://www.ipni.net/publication/nitrogen-en.